Which of the following are the three main inputs to the HMU that affect fuel flow to the combustor?

Which of the following are the three main inputs to the HMU that affect fuel flow to the combustor?

• A. Collective through the LDS; PCL thru the PAS; DEC thru the torque motor
• B. PCL through the PAS; DEC thru the torque motor; Collective through the LDS
• C. Collective through the PAS; PCL through the PAS; DEC thru the torque motor
• D. Collective through the LDS; DEC through the PAS; PCL through the torque motor

Answer: (A)

Fuel flow to the combustor is controlled by the HMU using three inputs that reflect what the pilot is asking for and what the engine must protect itself against: rotor load from the collective through the LDS, throttle demand from the PCL through the PAS, and a protective limit command from the DEC through the torque motor. The collective position is translated into a load demand signal via the LDS, telling the HMU how much fuel is needed to meet the rotor load. The PCL position is routed through the PAS so the HMU can interpret the pilot’s power setting as a torque request and adjust fuel accordingly. The DEC signal provides a limit or protection function and acts through the torque motor to trim or reduce fuel flow to prevent overspeed or overtorque. Together, these three inputs determine the fuel metering to the combustor.

Fuel flow to the combustor is controlled by the HMU using three inputs that reflect what the pilot is asking for and what the engine must protect itself against: rotor load from the collective through the LDS, throttle demand from the PCL through the PAS, and a protective limit command from the DEC through the torque motor. The collective position is translated into a load demand signal via the LDS, telling the HMU how much fuel is needed to meet the rotor load. The PCL position is routed through the PAS so the HMU can interpret the pilot’s power setting as a torque request and adjust fuel accordingly. The DEC signal provides a limit or protection function and acts through the torque motor to trim or reduce fuel flow to prevent overspeed or overtorque. Together, these three inputs determine the fuel metering to the combustor.